In the past, copper sheets have been used in various industrial fields as a material for connectors, electrodes, connecting terminals, terminals, relays, heat sinks and bus bars by utilizing the excellent electrical and heat conductivity thereof. However, since pure copper including C1100 and C1020 has low strength, the use per unit area is increased to ensure the strength and thus cost increases occur and weight increases also occur.
Cr—Zr copper (1% Cr-0.1% Zr—Cu), which is a solution heat-treating-aging.precipitation type alloy, is known as a high-strength and high-electrical conductivity copper alloy. However, in general, a rolled sheet using this alloy is manufactured through a heat treatment process in which a hot-rolled material is subjected to a solution heat treatment including re-heating at 950° C. (930° C. to 990° C.) and subsequent immediate quenching and is subjected to aging. Alternatively, a rolled sheet is manufactured through a heat treatment process in which after hot rolling, a hot-rolled material is subjected to plastic forming by hot or cold forging, heated at 950° C., rapidly quenched, and then subjected to aging. The high-temperature process of 950° C. not only requires significant energy, but oxidation loss occurs when the heating operation is performed in the air. In addition, because of the high temperature, diffusion easily occurs and the materials stick to each other, so an acid cleaning process is required.
For this reason, the heat treatment is performed at 950° C. in an inert gas or in vacuum, so the cost is increased and extra energy is also required. Further, although the oxidation loss is prevented by the heat treatment in an inert gas or the like, the sticking problem is not solved. Further, regarding the characteristics, crystal grains become coarse and problems occur in fatigue strength since the heating operation is performed at high temperatures. Meanwhile, in a hot rolling process in which the solution heat treatment is not performed, even when an ingot is heated to its solution heat temperature, the temperature of the material decreases during the hot rolling and along time is required to perform the hot rolling, so only very poor strength can be obtained. In addition, Cr—Zr copper requires special temperature management since a temperature condition range of the solution heat-treating is narrow, and if a cooling rate is also not increased, the Cr—Zr copper is not solution heat-treated. Meanwhile, when using Cr—Zr copper in a thin sheet, there is a method of performing the solution heat treatment by using a continuous annealing line in a stage of the thin sheet or a method of performing the solution heat treatment in a stage of the final punched product. However, when the solution heat treatment is performed by using a continuous annealing line, it is difficult to make a quenching state, and when the material is exposed to the high temperature such as 900° C. or 950° C., crystal grains become coarse and the properties become worse. When the solution heat treatment is performed on a final punched product, a productivity problem is caused and extra energy is also required. Moreover, since a large amount of active Zr and Cr is included, restrictions are imposed on the melting and casting conditions. As a result, excellent characteristics are obtained, but the cost is increased.
In the vehicle field using the copper sheets, while a decrease in the vehicle body weight is required to improve fuel efficiency, the number of components such as a connecting terminal, connector, relay and bus bar is increased due to the high-level informatization and the acquisition of electronic properties and hybrid properties (an increase in the number of electrical components) in a vehicle, and the number of heat sinks and the like for cooling the mounted electronic components is also increased. Accordingly, a copper sheet to be used is required to have a smaller thickness and higher strength. Naturally, in comparison to the case of home electric appliances and the like, regarding the vehicle usage environment, the temperature of the vehicle interior, as well as the engine room, increases in summer and enters harsh conditions. Further, since the usage environment is a high-current usage environment, it is particularly necessary to lower stress relaxation properties when a copper sheet is used in a connecting terminal, a connector and the like. The low stress relaxation properties mean that a contact pressure or spring properties of a connector and the like are not lowered in a usage environment of, for example, 100° C. In this specification, in a stress relaxation test to be described later, a low stress relaxation rate indicates “low” or “good” stress relaxation properties and a large stress relaxation rate indicates “high” or “bad” stress relaxation properties. It is preferable that a copper alloy rolled sheet has a low stress relaxation rate. As in vehicles, in the case of fittings such as a relay, terminal and connector, which are used in solar energy generation, wind power generation and the like, a high current flows therein, and thus high electrical conductivity is required and the usage environment thereof reaches 100° C. in some cases.
In addition, in many cases, due to the demands for high reliability, important electrical components are connected to each other by brazing, not soldering. Examples of a brazing filler material include Bag-7 (56Ag-22Cu-17Zn-5Sn alloy brazing filler material), described in JIS Z 3261, and a recommended brazing temperature thereof is in the high temperature range of 650° C. to 750° C. Accordingly, a copper sheet for use in connecting terminals and the like is required to have heat resistance of, for example, about 700° C.
In addition, for power modules and the like, a copper sheet for use in a heat sink or a heat spreader is joined to a ceramic or the like which is a base sheet. Soldering is employed for the above joining, but progress has been made regarding Pb-free solder and thus high-melting point solder such as Sn—Cu—Ag is used. In mounting a heat sink, a heat spreader and the like, it is required that not only does softening not occur but also that deformation and warpage do not occur and a small thickness is demanded in view of weight reduction and economy. Accordingly, a copper sheet is required to be not easily deformed even when exposed to high temperatures. That is, for example, a copper sheet is required to keep high strength even at about 350° C., which is higher than the melting point of the Pb-free solder by about 100° C., and to have resistance to deformation.
The invention is used in connectors, electrodes, connecting terminals, terminals, relays, heat sinks, bus bars, power modules, light-emitting diodes, lighting equipment components, members for a solar cell and the like, has excellent electrical and heat conductivity and realizes a small thickness, that is, high strength. In addition, when the invention is applied to connectors and the like, it is necessary to have good bendability and ductility such as bendability should be provided. Moreover, it is also necessary to have good stress relaxation properties. When simply increasing strength only, it is desirable that cold rolling is performed to cause work hardening. However, when a total cold rolling ratio becomes equal to or greater than 40%, and particularly equal to or greater than 50%, ductility including bendability becomes worse. Further, when a rolling ratio is increased, stress relaxation properties also become worse. Meanwhile, thin sheets are employed for the above-described using in connectors and the like, and in general, the thickness is 4 mm or equal to or smaller than 3 mm, or further equal to or smaller than 1 mm. In addition, since the thickness of a hot rolled material is in the range of 10 to 20 mm, a total cold rolling equal to or greater than 60%, and generally equal to or greater than 70% is required. In that case, an annealing process is generally added in the course of cold rolling. However, when causing the recrystallization by increasing the temperature in the annealing process, ductility is recovered, but strength becomes lower. In addition, when partially causing the recrystallization, although also depending on the relationship with the ratio of the subsequent cold rolling, ductility becomes poorer or strength becomes lower. In the invention of the present application, when a precipitation heat treatment is performed after cold rolling, precipitates of Co, P and the like to be described later are precipitated to strengthen the material, and at the same time, fine recrystallized grains or crystals (hereinafter, these crystal grains are referred to as fine crystals in this specification, and the fine crystals will be described later in detail) having a low dislocation density and a shape slightly different from that of recrystallized grains are formed partially around the original crystal grain boundaries to minimize a decrease in strength of the matrix and considerably improve ductility. In addition, by a series of processes, including causing work hardening by cold rolling with a rolling ratio not damaging ductility and stress relaxation properties and a final recovery heat treatment, high strength, high electrical and heat conductivity and excellent ductility are obtained.
A copper alloy is known which includes 0.01 to 1.0 mass % of Co, 0.005 to 0.5 mass % of P and the balance including Cu and inevitable impurities (for example, see JP-A-10-168532). However, such a copper alloy is also insufficient in both strength and electrical conductivity.